1. Field of the Invention
The field of art to which the claimed invention pertains is solid-bed adsorptive separation. More specifically, the claimed invention relates to a process for the separation of ethylbenzene from a feed mixture comprising ethylbenzene and at least one xylene isomer which process employs a solid adsorbent which selectively removes ethylbenzene from the feed mixture.
2. Description of the Prior Art
It is well known in the separation art that certain crystalline aluminosilicates can be used to separate hydrocarbons species from mixtures thereof. In particular, the separation of normal paraffins from branched chained paraffins can be accomplished by using the type A zeolite which have pore openings from 3 to about 5 Angstroms. Such a separation process is disclosed for example in U.S. Pat. Nos. 2,985,589 and 3,201,491. These adsorbents allow a separation based on the physical size differences in the molecules by allowing the smaller or normal hydrocarbons to be passed into the cavities within the crystalline aluminosilicate adsorbent, while excluding the larger or branched chain molecules.
U.S. Pat. Nos. 3,265,750 and 3,510,423 for example disclose processes in which larger pore diameter zeolites such as the type X or type Y structured zeolites can be used to separate olefinic hydrocarbons.
In addition to separating hydrocarbon types, the type X or type Y zeolites have also been employed in processes to separate individual hydrocarbon isomers. In the process described in U.S. Pat. No. 3,114,782 for example, a particular zeolite is used as an adsorbent to separate alkyl-trisubstituted benzene; and in U.S. Pat. No. 3,668,267 a particular zeolite is used to separate specific alkyl-substituted naphthalenes. The more well known selective adsorption processes however are the para-xylene separation processes. In U.S. Pat. No. 3,626,020 for example, a particular zeolite is used to separate para-xylene from a feed mixture comprising para-xylene and at least one other xylene isomer by selectively adsorbing para-xylene over the other xylene isomers. Para-xylene is one of the most commercially important aromatic hydrocarbon isomers. Its use in the manufacture of terephthalic acid which in turn is subsequently employed in the manufacture of various synthetic fibers is well known. One such fiber is Dacron which fiber is a trade-marked product of the duPont Company. The ever increasing demand for such fibers has resulted in a corresponding increase in the demand for para-xylene.
In contrast, the present invention relates to a process for the separation of ethylbenzene from a feed mixture comprising ethylbenzene and at least one other xylene isomer and is therefore distinguished from such xylene isomer separation processes. Additionally, in the process of this invention ethylbenzene is the selectively adsorbed extract component while the xylene isomers are less selectively adsorbed raffinate components.
We have found that adsorbents comprising type X structured zeolites containing at exchangeable cationic sites at least one cation selected from the group consisting of cations of Group I-A metals exhibit selectivity for ethylbenzene with respect to the xylene isomer thereby making separation of ethylbenzene from xylene isomers by solid-bed selective adsorption processes possible.
Ethylbenzene is used as a raw material in the production of styrene monomer. Ethylbenzene can be and is commercially produced from the alkylation of benzene with ethylene. The cost of and competing demands for necessary benzene and ethylene feed streams, have however prompted new efforts to recover ethylbenzene from various C.sub.8 aromatic feed streams which already contain ethylbenzene. Such feed streams for instance, include C.sub.8 aromatic extracts produced by a typical solvent extraction process from a pyrolysis gasoline or from a naphtha which has been reformed with a platinum-halogen-containing catalyst. Additionally C.sub.8 aromatic cuts of hydrogenated pyrolysis naphthas or reformates prepared by fractionation without solvent extraction contain varying amounts of ethylbenzene.
The particular utility of the process of our invention is that it offers a method for recovering ethylbenzene from a feed stream which already contains ethylbenzene.
Ethylbenzene can, of course, be separated from the xylene isomers by fractionation but because its boiling point is within about 4.degree. F. of that of para-xylene, the fractionation can be achieved only with the more intricate super-fractionators. Typical ethylbenzene fractionators contain 300 to 400 actual trays and require about a 25-50 to 1 reflux to feed ratio. The process of our invention therefore offers a competitive alternative to the separation of ethylbenzene by super-fractionation.